Non-aqueous electrolyte secondary battery

ABSTRACT

A solid solution represented by the formula (1): Li x M 1   y M 2   z  is used as a negative electrode active material for a non-aqueous electrolyte secondary battery comprising a chargeable and dischargeable positive electrode, a non-aqueous electrolyte and a chargeable and dischargeable negative electrode. In the formula (1), M 1  represents at least one element selected from the group consisting of Ti, Zr, Mn, Co, Ni, Cu and Fe, and M 2  represents at least one element selected from the group consisting of Si and Sn, and 0≦x&lt;10, 0.1≦y≦10 and z=1.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of Internaional Application No.PCT/JP00/04283, filed Jun. 28, 2000, the disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a non-aqueous electrolyte secondarybattery. More particularly, the present invention relates to anon-aqueous electrolyte secondary battery having a highly reliablenegative electrode with a high electric capacity and with the growth ofdendrite suppressed.

BACKGROUND ART

High voltage and high energy density can be expected from non-aqueouselectrolyte secondary batteries using lithium or a lithium compound inthe negative electrode. Positive electrode active materials fornon-aqueous electrolyte secondary batteries that have been known includeoxides and chalcogens of transition metals such as LiMn₂O₄, LiCoO₂,LiNiO₂, V₂O₅, Cr₂O₅, MnO₂, TiS₂ and MoS₂. Those compounds have a layeredor tunneled crystal structure that allows intercalation anddeintercalation of lithium ions. As to the negative electrode activematerial, on the other hand, there are many previous studies on metalliclithium. However, the use of metallic lithium causes dendrite to depositon the surface of lithium during charging, which reducescharge/discharge efficiency. In addition, internal short-circuit iscaused if dendrites come in contact with the positive electrode.

As a solution to those problems, the use of a lithium alloy such aslithium-aluminum alloy, which not only suppresses the growth of dendritebut also can absorb lithium therein and desorb it therefrom in thenegative electrode, has been studied. However, in case the lithium alloyis used, the repeating of deep charging and discharging causespulverization of the electrode, presenting a problem concerning thecycle life characteristics.

In recent years, a highly safe carbon material capable of reversiblyabsorbing and desorbing lithium and excellent in cycle lifecharacteristics has been used in the negative electrode, althoughsmaller in capacity than metallic lithium or lithium alloy. In anattempt to further increase the capacity of the non-aqueous electrolytesecondary battery, studies have been made about application of oxides tothe negative electrode. For example, it is suggested in JapaneseLaid-Open Patent Publications Hei 7-122274 and Hei 7-235293 thatcrystalline SnO and SnO₂ may serve as negative electrode materialshaving high capacities than the conventional oxide WO₂. It is alsoproposed in Japanese Laid-Open Patent Publication Hei 7-288123 thatnon-crystalline oxide such as SnSiO₃ or SnSi_(1−x)P_(x)O₃ is used in thenegative electrode in order to improve the cycle life characteristics ofthe battery. However, no sufficient characteristics have been obtainedyet.

In view of the circumstance described above, it is the object of thepresent invention to provide a non-aqueous electrolyte secondary batteryhaving a high capacity and excellent charge/discharge cycle lifecharacteristics in which no dendrite grows because the negativeelectrode absorbs lithium upon charging.

DISCLOSURE OF INVENTION

The present invention relates to a non-aqueous electrolyte secondarybattery comprising a chargeable and dischargeable positive electrode, anon-aqueous electrolyte and a chargeable and dischargeable negativeelectrode, wherein the negative electrode has a solid solution, as anactive material, the solid solution being represented by the formula(1):

Li_(x)M¹ _(y)M² _(z)  (1)

wherein M¹ represents at least one element selected from the groupconsisting of Ti, Zr, Mn, Co, Ni, Cu and Fe, and M² represents at leastone element selected from the group consisting of Si and Sn, and wherein0≦x<10, 0.1≦y≦10 and z=1.

The average particle size of the solid solution represented by theformula (1) is preferably 0.5 to 2.3 μm. And the average crystal grainsize of the solid solution represented by the formula (1) is preferably0.05 to 0.13 μm.

The above-mentioned negative electrode contains preferably 5 to 50 partsby weight of carbon material per 100 parts by weight of the solidsolution represented by the formula (1).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a test cell used for evaluating theelectrode characteristics of a negative electrode used in a non-aqueouselectrolyte secondary battery of the present invention.

FIG. 2 is a cross-sectional view of a cylindrical battery as an exampleof the non-aqueous electrolyte secondary battery of the presentinvention.

FIG. 3 depicts X-ray diffraction patterns of the powder of Fe₂Sn as anexample of the solid solution in accordance with the present invention,and the powder of α-Fe.

FIG. 4 depicts X-ray diffraction patterns of negative electrodes, inwhich Fe₂Sn as an example of the solid solution in accordance with thepresent invention is used as an active material, in the initial state(a), in the first charged state (b), in the first discharged state (c)and in the discharged state after 500 cycles (d).

BEST MODE FOR CARRING OUT THE INVENTION

The present invention relates to a non-aqueous electrolyte secondarybattery having a chargeable and dischargeable positive electrode, anon-aqueous electrolyte comprising a non-aqueous solvent containinglithium salt dissolved therein and a chargeable and dischargeablenegative electrode, wherein the negative electrode contains, as anactive material, an alloy represented by the formula (1): Li_(x)M¹_(y)M² _(z). This alloy is a solid solution in which M² is dissolved inthe crystal structure of M¹, or M¹ is dissolved in the crystal structureof M².

M¹ in the formula (1) is at least one element selected from the groupconsisting of Ti, Zr, Mn, Co, Ni, Cu and Fe. That is, M¹ may be acombination of two or more elements. In view of the structural stabilityof the solid solution, however, it is desirable to use one of themalone.

As to M² in the formula (1), at least one element selected from thegroup consisting of Si and Sn is used, because excellentcharge/discharge characteristics can be obtained therefrom in thebattery.

Herein, 0≦x<10, and to suppress dendrite effectively, it is desirablethat 0≦x<5.

And 0.1≦y≦10. When y is less than 0.1, the structure of the solidsolution is unstable and the solid solution used as an active materialdeteriorates in the charge/discharge reaction. When y is more than 10,on the other hand, the battery capacity is decreased.

And z=1. It is noted that the value of x changes with absorbing ordesorbing Li in the charge/discharge reaction of the battery.Immediately after a battery is fabricated, x is generally 0.

It is desirable that the crystal grain of the solid solution is 0.05 to0.13 μm in average grain size. Because when the grain size is small likethat, many grain boundaries are formed in the solid solution. And thegrain boundaries function to suppress the expansion of the solidsolution down to a minimum when lithium is absorbed therein.

It is also noted that the smaller the particle size of the solidsolution as the active material is, the larger the specific surface areais and the more improved the efficiency of battery reaction is. But whenthe particle size is too small, there arise such problems as difficultyin handling and side reaction between the electrolyte and the solidsolution, and therefore it is desirable that the average particle sizeof the solid solution is 0.5 to 2.3 μm.

Furthermore, it is desirable that the negative electrode contains suchcarbon materials as graphite, low-crystalline carbon materials andacetylene black, and the content thereof is preferably 5 to 50 parts byweight per 100 parts by weight of the solid solution. When the solidsolution and the carbon material are mixed and used in the negativeelectrode, the retention of electrolyte in the negative electrode isimproved and the charge/discharge cycle life characteristics is alsoimproved. However, too much carbon material would make it difficult toget the most of the feature that the active material has a high energydensity, and to increase the capacity of the battery.

The solid solution can be synthesized by any of the following exemplaryprocesses: mechanical alloying, liquid quenching, ion beam sputtering,vacuum evaporation, plating and chemical vapor deposition (CVD). Amongthem, the solid solution used in the present invention can be producedeasily by the liquid quenching process or mechanical alloying process inparticular. In the liquid quenching process, for example, the molten rawmaterial can be quenched by a single roll at a rate of 10⁵ to 10⁶K/second, and a solid solution having a micro crystal grain can beobtained. The mechanical alloying process also can produce a microcrystal grain and a phase of solid solution that can not be obtained bythe conventional thermal technique.

The solid solutions represented by the formula (1) include, for example,FeSn₂, FeSn, Fe₂Sn, Fe₃Sn, CuSn, Cu₂Sn, Cu₃Sn, Cu₆Sn₅, TiSn, Ti₂Sn,Ti₃Sn, ZrSn, Zr₂Sn, MnSn, MnSn₂, Mn₂Sn, Mn₃Sn, CoSn, CoSn₂, Co₂Sn,Co₃Sn, NiSn, NiSn₂, Ni₂Sn, Ni₃Sn, FeSi, Fe₂Si, Fe_(2.5)Si, Fe_(2.3)Si,Fe₃Si, CuSi, Cu₂Si, Cu₃Si, TiSi, TiSi₂, Ti₂Si, Ti₃Si, ZrSi, Zr₂Si, MnSi,MnSi₂, Mn₂Si, Mn₃Si, CoSi, Co₂Si, Co₃Si, NiSi, NiSi₂ and Ni₂Si.

In the phase where M² is dissolved in the crystal structure of M¹, or M¹is dissolved in the crystal structure of M², M¹ surrounding M² is firmlybonded to M², or M² surrounding M¹ is firmly bonded to M¹. Therefore, itis considered that such a solid solution can form a Li—M² alloy having avery small crystal grain by absorbing lithium. In a negative electrodeusing the solid solution as an active material, therefore, the growth ofdendrite is suppressed, and furthermore, active M² is not isolated anddoes not float with ease, thus maintaining the crystal structure andeffectively improving the cycle life characteristics of the negativeelectrode.

The negative electrode that is used in the non-aqueous electrolytesecondary battery of the present. invention is prepared in the followingway, for example. First, 5 to 50 parts by weight of carbon material with100 parts by weight of the solid solution represented by the formula(1), an appropriate quantity of a binder and an appropriate quantity ofan electrolyte or a solvent are mixed. And the mixture is molded into aspecific form. The carbon material used here is graphite, acetyleneblack or low-crystalline carbon material, for example. As to the binder,poly(vinylidene fluoride), SBR (stylene-butadiene copolymer rubber),polyethylene, polytetrafluoroethylene or the like is preferably used.

The non-aqueous electrolyte secondary battery of the present inventioncan be fabricated in the same way as conventional batteries except thatthe aforementioned negative electrode is used. Therefore, it is possibleto use chargeable and dischargeable positive electrodes and non-aqueouselectrolytes that have been used in the conventional non-aqueouselectrolyte secondary batteries can be used with no particularrestriction.

In the following, the present invention will be described moreconcretely on the basis of examples. But the present invention is notlimited thereto.

First, there will be described the test cell shown in FIG. 1 and thecylindrical battery shown in FIG. 2 used in the examples and comparativeexamples given below. The test cell was used for evaluation of theelectrode characteristics of the negative electrode in which the solidsolution in accordance with the present invention is used as the activematerial. The cylindrical battery was used for evaluation of the cyclelife characteristics of the battery with a negative electrode in whichthe solid solution in accordance with the present invention is used asthe active material.

Test Cell

A mixture was prepared by mixing 8.5 g of a negative electrode activematerial (solid solution), 1 g of graphite powder as a conductive agentand 0.5 g of polyethylene powder as a binder. Then, 0.1 g of the mixturewas pressure-molded into a disk with a diameter of 17.5 mm to obtain atest electrode 1. Then, the test electrode 1 was placed in a case 2 asshown in FIG. 1. And a separator 3 made of microporous polypropylene wasplaced thereon. Then, the non-aqueous electrolyte was filled into case2. The electrolyte used here was prepared by dissolving 1 mol/l oflithium perchlorate (LiClO₄) in a mixed solvent of ethylene carbonateand dimethoxyethane in an equivolumetric ratio. And the case 2 wassealed with a sealing plate 6 having a metallic lithium 4 in the form ofa disk with a diameter of 17.5 mm attached inside thereof, and apolypropylene gasket 5 on the periphery thereof to complete a test cell.

Cylindrical Battery

The negative electrode active material (solid solution), graphite powderas a conductive agent and polytetrafluoroethylene as a binder were mixedin a ratio of 60:3:10 by weight. Then, to this mixture, a petroleum typesolvent was added and a paste was obtained. The paste thus obtained wasapplied on a copper core and dried at 100° C. to produce a negativeelectrode plate.

Meanwhile, the positive electrode active material LiMn_(1.8)Co_(0.2)O₄was synthesized by mixing Li₂CO₃, Mn₃O₄ and CoCO₃ in a predeterminedmolar ratio, followed by heating at 900° C. Of the active material thusobtained, particles not larger than 100 mesh in size were sieved out.Then, 100 g of the positive electrode active material, 10 g of graphiteas a conductive agent and 8 g (resin content) of aqueous dispersion ofpolytetrafluoroethylene as a binder were mixed. To this mixture, purewater was added to obtain a paste. The paste thus obtained was appliedon a titanium core, followed by drying and rolling, to produce apositive electrode plate.

Using the positive electrode and negative electrode thus obtained, acylindrical battery was assembled in the following way. Between apositive electrode plate 8 having a spot-welded positive electrode lead7 made of the same material as that of the core and a negative electrodeplate 10 having a spot-welded negative electrode lead 9 made of the samematerial as that of the core, a band-like separator 11 made of a porouspolypropylene wider than the two electrode plates was placed. And thewhole was wound up into an electrode group. Then, the electrode groupwas inserted into a battery case 14, with a polypropylene insulatingplate 12 and 13 placed at the top and bottom of the electrode group,respectively. After a step was formed at the upper part of the batterycase 14, a non-aqueous electrolyte prepared by dissolving 1 mol/l oflithium perchlorate (LiClO₄) in a mixed solvent of ethylene carbonateand dimethoxyethane in an equivolumetric ratio was injected into thebattery case 14. Then, the opening of the battery case 14 was sealedusing a sealing plate 15.

EXAMPLES 1 to 45

Solid solutions having compositions represented by the formula (1),where M² is dissolved in the crystal structure of M¹, or M¹ is dissolvedin the crystal structure of M², were prepared in the procedure asmentioned below. And using these as negative electrode active material,the above-mentioned test cells and cylindrical batteries were fabricatedand evaluated.

First, solid solutions having compositions shown in Table 1 wereprepared. The raw materials M¹ and M² were selected with one element foreach of them, and mixed in a predetermined molar ratio. Then, themixture was placed in a pot mill made of stainless steel with aninternal volume of 0.5 liters having 20 stainless steel balls (½ inch indiameter) accommodated therein, and the mill was sealed in an argonatmosphere. This mill was rotated at 60 rpm for one week to obtain anintended solid solution. The solid solutions thus obtained were allbetween 0.5 and 2.3 μm in average particle size. And the average crystalgrain size of any of the solid solutions calculated from an X-raydiffraction pattern was between 0.05 and 0.13 μm.

Among the obtained solid solutions, the X-ray diffraction pattern ofFe₂Sn is shown in FIG. 3. FIG. 3 shows that Fe₂Sn has a single solidsolution phase and that no peak attributable to Fe and Sn is present. InFIG. 3, no peak attributable to a Fe—Sn type intermetallic compound isobserved, either. It was found, from a detailed analysis of peak shiftin the X-ray diffraction pattern, that in this solid solution, Sn atomswere infiltrated in the crystal structure of Fe, which is abody-centered cubic (bcc) structure, but that the bcc structure wasmaintained. If it is supposed that all the Sn atoms in Fe₂Sn areinfiltrated into the crystal structure of Fe and that the bcc structureis maintained, the position of a peak attributable to its (100) crystalface is calculated at 43°. This value agrees with the actual measurementobtained from FIG. 3. This also indicates that Fe₂Sn in the example is asolid solution where Sn is infiltrated into the crystal structure of Fe.It was also confirmed that the solid solution in any of the otherexamples is a solid solution where M² is dissolved in M¹, or M¹ isdissolved in M².

Then, using the above-mentioned solid solutions in the test electrode,test cells were fabricated. Then at a constant current of 2 mA, the testcell was subjected to cathode polarization (corresponding to chargingwhen the test electrode was seen as a negative electrode) until theelectric potential of the test electrode became 0 volt against thecounter electrode of metallic lithium. Then, the test cell was subjectedto anode polarization (corresponding to discharging when the testelectrode was seen as a negative electrode) until the electric potentialof the test electrode became 1.5 volts against the counter electrode ofmetallic lithium. After that, cathode polarization and anodepolarization were repeated. The first discharge capacity per 1 g ofactive material of the test electrode is shown in Table 1.

After the cathode polarization, and after the cathode polarization andanode polarization were repeated 10 times, respectively, all the testcells were disassembled and the test electrodes were taken out andobserved. No deposition of metallic lithium was observed on the surfaceof any of the electrodes. From this, it is shown that it is difficultfor dendrite to grow on the surface of the electrode in which the solidsolution of the example is used as the active material. Furthermore,when the test electrodes were put to elemental quantitative analysisafter the cathode polarization, the quantity of lithium contained in theactive material was within the range of x (0≦x<10) in the formula (1) inany of the test electrodes.

Next, cylindrical batteries were fabricated using the above-mentionedsolid solutions in the negative electrodes. And the batteries thusobtained were subjected to the repetition of a charge/discharge cycle at30° C. Then, the capacity maintenance rates at the 100th cycle to thatat the first cycle were worked out. In this test, the charge/dischargecurrent was 1 mA/cm², and the charge/discharge voltage was between 2.6and 4.3 volts. The results are shown in Table 1.

TABLE 1 Capacity Discharge Maintenance Example No. Composition Capacity(mAh/g) rate (%) 1 FeSn 670 94 2 FeSn₂ 920 91 3 Fe₂Sn 530 97 4 Fe₃Sn 41099 5 CuSn 520 94 6 Cu₂Sn 460 96 7 Cu₃Sn 350 99 8 TiSn 500 96 9 Ti₂Sn 42097 10 Ti₃Sn 370 99 11 ZrSn 480 98 12 Zr₂Sn 400 98 13 MnSn 490 92 14MnSn₂ 620 85 15 Mn₂Sn 400 99 16 Mn₃Sn 360 100  17 CoSn 570 91 18 Co₂Sn490 95 19 Co₃Sn 400 99 20 NiSn 540 92 21 Ni₂Sn 460 95 22 Ni₃Sn 390 99 23FeSi 520 94 24 Fe₂Si 460 96 25 Fe_(2.5)Si 350 99 26 Fe_(2.3)Si 500 96 27Fe₃Si 420 97 28 CuSi 370 99 29 Cu₂Si 480 98 30 Cu₃Si 400 98 31 TiSi 35094 32 Ti₂Si 360 92 33 Ti₃Si 580 94 34 ZrSi 520 93 35 Zr₂Si 410 95 36MnSi 590 92 37 MnSi₂ 460 93 38 Mn₂Si 570 91 39 Mn₃Si 490 95 40 CoSi 54092 41 Co₂Si 460 95 42 Co₃Si 390 99 43 NiSi 400 99 44 NiSi₂ 580 100  45Ni₂Si 470 95

Comparative Examples 1 to 4

The same procedure as in the preceding examples was followed except thatthe previously reported Fe-Sn type intermetallic compound (J. R. Dahn etal., Journal of Electrochemical Society, 146 (2), 414-422 (1999)) wasused, and the evaluation was also conducted in the same manner.

As the Fe—Sn type intermetallic compounds, FeSn₂, FeSn, Fe₃Sn₂ andFe₅Sn₃ shown in Table 2 were used. Those intermetallic compounds wereprepared using the high-speed ball mill as mentioned in the previousreport, followed by heat treatment. The evaluation results are shown inTable 2.

The intermetallic compounds in the above-mentioned comparative exampleswere all between 1.8 and 26 μm in average particle size. It is believedthat the primary particles are aggregated in the heat treatment. Also,because of the heat treatment, any of the intermetallic compounds in allthe comparative examples had a large average grain size of 0.37 to 1.9μm.

TABLE 2 Capacity Comparative Discharge Maintenance Example No.Composition Capacity (mAh/g) rate (%) 1 FeSn₂ 800 1 2 FeSn 250 3 3Fe₃Sn₂ 150 4 4 Fe₅Sn₃ 100 5

The results in Table 1 and Table 2 show that the batteries of theexamples in which the above-mentioned solid solutions were used in thenegative electrodes are high in capacity and capacity maintenance rateand excellent in cycle life characteristics. In comparison, many of thebatteries of the comparative examples are very low in capacitymaintenance rate and insufficient in capacity.

In FIG. 4, there are shown X-ray diffraction patterns of the negativeelectrodes, in which a solid solution having a composition of Fe₂Sn isused as an active material, in the initial state (a), in the firstcharged state (b), in the first discharged state (c) and i n thedischarged state after 500 cycles (d). FIG. 4 shows that the repetitionof charge/discharge reactions does not cause any shift of the peakattributable to the crystal structure of the solid solution. In anydiffraction pattern in FIG. 4, no peak is found that indicates theformation of a Li—Sn alloy. As a reason why the solid solutions in theexamples are high incapacity and excellent in cycle life characteristicslike described above, it can be pointed out that even after the chargingand discharging are repeated, the solid solutions maintain their initialcrystal structures.

In the foregoing examples, the cylindrical batteries have beendescribed, but the effect of the present invention is th e same whencoin type, rectangular type, or flat type battery is assembled. In theforegoing examples, the solid solutions were prepared by the mechanicalalloying process, but the same effects can be obtained if the solidsolutions are prepared by the liquid quenching, ion beam sputtering,vacuum evaporation, plating or CVD process. It is also noted that in theforgoing examples, LiMn_(1.8)Co_(0.2)O₄ was used in the positiveelectrodes, but the same effects can be obtained with LiMn₂O₄, LiCoO₂,LiNiO₂ or the like.

Industrial Applicability

As described above, according to the present invention, ahighly-reliable non-aqueous electrolyte secondary battery with a highenergy density and free from dendrite-induced short-circuit can beobtained because a negative electrode high in capacity and excellent incycle life characteristics is used.

What is claimed is:
 1. A non-aqueous electrolyte secondary batterycomprising a chargeable and dischargeable positive electrode, anon-aqueous electrolyte, and a chargeable and dischargeable negativeelectrode, the negative electrode including, as an active material, asolid solution represented by formula (1): Li_(x)M¹ _(y)M² _(z)  (1)wherein M¹ represents at least one element selected from the groupconsisting of Ti, Zr, Mn, Co, Ni, Cu and Fe, M² represents at least oneelement selected from the group consisting of Si and Sn, 0≦x<10,0.1≦y≦10, and z=1, and wherein the solid solution represented by formula(1) has an average crystal grain size of 0.05 to 0.13 μm.
 2. Thenon-aqueous electrolyte secondary battery in accordance with claim 1,wherein said solid solution represented by the formula (1) has anaverage particle size of 0.5 to 2.3 μm.
 3. The non-aqueous electrolytesecondary battery in accordance with claim 1, wherein said negativeelectrode contains 5 to 50 parts by weight of carbon material per 100parts by weight of said solid solution represented by the formula (1).4. A non-aqueous electrolyte secondary battery comprising a chargeableand dischargeable positive electrode, a non-aqueous electrolyte, and achargeable and dischargeable negative electrode, said negative electrodeincluding, as an active material, a solid solution represented by theformula (1): Li_(x)M¹ _(y)Sn_(z)  (1) wherein M¹ represents at least oneelement selected from the group consisting of Ti, Zr, Mn, Co, Ni, Cu andFe, 0≦x<10, 0.1≦y≦10, and z=1.
 5. The non-aqueous electrolyte secondarybattery in accordance with claim 4, wherein the solid solutionrepresented by formula (1) has an average particle size of 0.5 to to 2.3μm.
 6. The non-aqueous electrolyte secondary battery in accordance withclaim 4, wherein the solid solution represented by formula (1) has anaverage crystal grain size of 0.05 to 0.13μm.
 7. The non-aqueouselectrolyte secondary battery in accordance with claim 4, wherein thenegative electrode contains 5 to 50 parts by weight of carbon materialper 100 parts by weight of the solid solution represented by formula(1).